The present invention relates to a circuit for an artificial, in particular implantable, cardiac pacemaker which is suitable for different modes of operation.
Artificial pacemakers are known which in cases of heart weakness or disorders in the natural heart rhythm take over the stimulation of the heart and take care that in spite of the presence of a pathological defect in the area of the natural function control of the heart it will be able to perform its life sustaining task.
Such pacemakers include those of the demand-mode type in which the time duration of such pulses is preferably determined by the refractory period, and those of the auricle controlled type in which the time duration is determined by the P-O delay. The refractory period is that period of time during which no stimulation pulse is actuated, even if an input signal should appear. The P-Q delay is the period of delay, which corresponds to the transit time of the physiological excitation from the auricle to the ventricle.
The life of a patient depends to a decisive degree on the orderly operation of the pacemaker. It must be assured that the device is continuously operable under any conditions without external access for the longest possible time.
Interruption in the operation of a pacemaker may occur, in principle, as a result of two different causes:
1. by malfunction due to the unpredictable failure of any component; PA1 2. by exhaustion of the source of energy.
The former type of interruption occurs unexpectedly, so that it leads to a dangerous situation for the patient which must be avoided under all circumstances. Efforts have therefore been made to make the reliability of the circuits used in pacemakers so great that the risk of unexpected failure is reduced practically to zero. Reliability factors have been attained which exceed the values required in space science.
In order to further reduce the risk of failure, it has been attempted to design the circuitry employed so that failure of a single component cannot lead to complete malfunction of the entire device or so that the impending failure of a component will become evident in time by a change in the operating parameters of the device.
The realization of high reliability, i.e. dependable assurance against unexpected operating malfunctions, involves considerable expenditures since the selection of components and the controls imposed during various manufacturing stages must be effected with extreme care.
The probability of an unexpected malfunction in operation generally increase with the length of time during which the device has been in use. At a certain point in time, therefore, the operating dependability of the pacemaker has decreased to such a point that the required reliability is no longer assured. Thus usefulness of the device has come to an end.
This point in time depends not only on the increase in the statistical failure rate but also on a number of predictable events, such as the attack of body fluids on the housing and the deterioration of its properties due to aging of the components. Through the use of high quality materials and components it is today possible to secure many years of proper operation from an artificial pacemaker. It has been found that an increase in the operating dependability generally also leads to an increase in its expected service life and thus its possible period of use, since both values are linked together.
The second group of operating malfunctions presents a lesser risk for the patient since the exhaustion of the energy sources occurs at regular intervals and can be determined in time by appropriate measuring devices. Even an unscheduled premature exhaustion of an energy source can be determined by check-up examinations which can take place at long intervals because the period of time between the first sign of reduction in the performance of the energy source and its final failure is generally sufficiently long. It is, of course, desired to make the operating life of the energy sources in implantable pacemakers as long as possible since a change in energy source always requires explanation connected with surgical procedure which should always be performed as rarely and at as long intervals as possible in each case.
Presently the energy sources for implantable artificial pacemakers are preferably primary chemical elements whose service life, which depends on the current consumption of the electrical pacemaker circuit employed, is at least several years. For the recently employed lithium silver chromate elements, the expected service life is already about 8 to 9 years. For economic reasons, the possible life of the pacemaker should be at least equal to, and preferably greater than, the maximum period of operation of the batteries.
In addition to longer service life, lithium silver chromate elements have the additional advantage of a greater energy density compared to the mercury oxide zinc elements which were previously used most commonly. The potential of two to three volts of the lithium elements is above the values of the primary elements which were previously used most frequently. Since, however, to promote greater redundancy which decreases the chances for failure, it is preferred to connect the lithium cells in parallel instead of in series as was previously done, there is in the end less potential available to supply the pacemaker circuit with current. The previously employed circuits have the drawback that they have poorer operating characteristics at lower supply potentials than they have at higher potentials.
It is thus obvious that the existing requirements led to the development of devices in the pacemaker art which meet the highest demands with respect to expected service life and reliability but which do involve considerable expenditures.
Pacemakers offer decisive advantages over treatment with medications and in order to allow pacemakers to be even more broadly applicable, there exists the need to reduce the costs of manufacture and use without reducing the level of the operating requirements which they must satisfy.
One development in this direction, to reduce the costs of use of pacemakers, involves making the pacemakers to be reusable. Experience has shown that pacemakers are predominantly implanted in patients who are already at an advanced age or those whose heart damage coincides with other organic damage. Thus it may happen that the probable service life of the pacemaker exceeds the life expectancy of the patient. It has therefore been made possible, in principle, to reuse a pacemaker after disinfection, i.e. to implant it in another patient. Since, however, the pathology connected with heart rhythm disorders varies from one person to another, there often exists the necessity to adapt a pacemaker, before it is reimplanted, to the particular requirements of the new user unless there exists a store of a large number of devices meeting the large variety of different pathological symptoms.
It also happens that the pathological behavior of the heart of one and the same patient changes in the course of time requiring a corresponding modification of the operating parameters of the pacemaker. Thus, for example, the threshold value of the heart may change with time. The likelihood that such a change will occur and that the pacemaker must be adapted to it increases with the length of time the device is implanted in a patient to stimulate his heart. Therefore, the necessity of having to change one or more of the operating parameters will occur more frequently the longer the pacemaker has been in service.
Such adaptation of operating parameters to changes occurring in the patient may be effected, for example, at the time of the explanation which is in any event required to permit battery replacement. It is also possible, however, to adjust the mode of operation of a pacemaker while it is in the implanted state, by means of remote control. If means for subsequent variation of the operations parameters of a pacemaker are not provided, it will be necessary for the patient, if his pathological symptoms change, to receive a new device before the maximum service life of his old device has been utilized.
Pacemakers of uniform types which can be adapted to various types of heart rhythm disorders can be produced more economically and with lower operating and storage costs than a plurality of different types of devices. Since manufacturing costs represent a high proportion of the total costs of a pacemaker, a reduction in such costs can significantly reduce a patient's expenses.
German Offenlegungsschrift [Laid-open Application] No. 2,163,482 discloses a pacemaker circuit which permits fixing of the heart threshold value without surgical procedure. The procedure circuit is associated with switching mechanisms which permit remote setting of the amplitude of the pacemaker output in dependence on the heart threshold value determined by the physician. In the pacemaker circuit described in the above-mentioned application it is possible to subsequently adapt the device to a changed threshold value of a patient but it is not possible to subsequently adapt it to changes in the basic pathological symptoms which would require, for example, a transition from negative R wave control to positive R wave control. Additionally, that circuit does not permit of a reduction in production costs since a variation of the output voltage of the pacemaker, as required upon a change in the threshold voltage of the heart, can be accomplished simply by changing a few resistance values so that no need exists for a completely new circuit design.